An electrostatic discharge (ESD) is an undesired electric discharge that can occur between two conductors, such as two pins of a semiconductor device. Typically, such a discharge can cause immediate and permanent damage to an electronic circuit. In order to mitigate the effects of electrostatic discharges, ESD protection circuits are often incorporated in electronic circuits. An example of an ESD protection circuit is provided by United States Patent Application publication US 2011058293, which is designed for sensing ESD currents of parallel pins and to switch off the semiconductor device when the ESD current exceeds a threshold value.
International patent application publication WO 2014/041388 also discloses in FIG. 4a a semiconductor device provided with an electrostatic discharge protection (ESD) protection device. This ESD protection device comprises a PNP-NPN transistor pair which together constitute a silicon controlled rectifier (SCR). A further PNP transistor and the PNP transistor of the SCR together constitute a current mirror. Both the SCR and the further transistor are connected to ground. Such an ESD protection device has a single breakdown voltage. The ESD protection comprises a current mirror and two SCRs connected in series. This ESD protection device also has a single, but higher, breakdown voltage.
For certain applications, such as battery monitoring and control applications, multiple cumulative voltages should be protected. For instance, a stack of batteries may be protected using a stack of ESD protection devices, as disclosed in U.S. Pat. No. 8,294,422, for example. A serious disadvantage of such a stack of ESD devices is that their series resistance at breakdown adds up. That is, at breakdown the series resistance of the stack of ESD devices is the sum of the breakdown resistances of the individual devices. When using four or five ESD protection devices in series, this overall resistance may limit the effectiveness of the ESD protection devices. A possible solution is to increase the size of the ESD components to reduce their resistance. However, in integrated circuits such an enlargement of components is often undesirable due to the limited dimensions of the integrated circuit.